The present invention relates to an optical waveguide circuit such as an arrayed waveguide grating used for optical transmissions, etc.
Recently, optical wavelength multiplexing transmissions have been researched and studied as a method for remarkably increasing transmission capacity, some of which have been used in practical applications. Such an optical wavelength multiplexing transmission is used to transmit, for example, a plurality of light having different wavelengths through multiplexing. In such an optical wavelength multiplexing transmission system, in order to pick up light wavelength by wavelength at the light receiving side from a plurality of transmitted light, it is indispensable that an optical transmission element, etc., which can transmit only light having predetermined wavelengths is provided in the system.
As one example of optical transmission elements, there is an arrayed waveguide grating (AWG: Arrayed Waveguide Grating) as shown in, for example, FIG. 6. The arrayed waveguide grating is such that it has a waveguide construction, as shown in the same drawing, on a substrate 11. The waveguide construction of the arrayed waveguide grating is as shown below. That is, an input side slab waveguide 13 is connected to the emission side of incidence waveguides 12 as one or more optical input waveguides juxtaposed to each other. And, a plurality of juxtaposed waveguides 14 are connected to the emission side of the input side slab waveguide 13. An output side slab waveguide 15 acting as the second slab waveguides is connected to the emission side of the plurality of arrayed waveguides 14. Emission waveguides 16 acting as a plurality of juxtaposed optical output waveguides are connected to the emission side of the output slab waveguide 15.
The arrayed waveguides 14 propagate light introduced from the input side slab waveguide 13, and are formed so as to have lengths different from each other. Also, the emission waveguides 16 are provided so as to correspond to the number of signal light, divided by, for example, an arrayed waveguide grating, having wavelengths different from each other. In addition, a number (for example, 100 lines) of arrayed waveguides 14 are usually provided. However, in the same drawing, in order to simplify the drawing, the number of the respective waveguides 12, 14, and 16 is simplified for illustration.
For example, transmission side optical fibers are connected to the incidence waveguides 12, into which wavelength multiplexed lights are introduced. Subsequently, light which is introduced into the input side slab waveguide 13, passing through the incidence waveguides 12, is diffracted by the diffraction effect thereof, and made incident into a plurality of respective arrayed waveguides 14, wherein the light propagates through the respective arrayed waveguides 14.
Light which propagated through the respective arrayed waveguides 14 reaches the output side slab waveguide 15, and is further condensed at the emission waveguides 16 for output. Also, since the lengths of the respective arrayed type waveguides 14 differ from each other, a shift occurs in the phases of individual lights after they propagated through the respective arrayed waveguides 14. And, the phase front of a light from the arrayed waveguide is inclined in compliance with the quantity of the tilt, wherein the position of condensing the light is determined by the angle of the inclination, and the light condensing positions of light having different wavelengths differ. By forming the emission waveguides 16 at the condensing positions of light having different wavelengths, it is possible to output light having different wavelengths from the emission waveguides 16 wavelength by wavelength.
For example, as shown in the same drawing, as wavelength multiplexed light having wavelengths xcex1, xcex2, xcex3, . . . xcexn (n: an integral number) is inputted from one incidence waveguide 12, the light is diffracted by the input side slab waveguide 13. And, it reaches the arrayed waveguides 14, and as described above, is condensed at different positions on the basis of wavelength, passing through the output side slab waveguide 15. Light having different wavelengths is made incident into different emission waveguides 16, and are outputted from the emission ends of the emission waveguides 16, passing through the respective emission waveguides 16. And, since optical fibers for light output are connected to the emission ends of the respective emission waveguides 16, it is possible to pick up light of respective wavelengths via the optical fibers.
In the arrayed waveguide grating, the wavelength resolution of the diffraction grating is proportional to a difference (xcex94L) in length of the respective arrayed waveguides 14. Therefore, by designing the xcex94L so as to become larger, it will become possible to multiplex and demultiplex wavelength multiplexed light having narrow wavelength intervals, which could not be achieved in prior arts. That is, the arrayed waveguide grating can achieve light multiplex and demultiplex functions (functions to multiplex and demultiplex a plurality of light signals having a wavelength interval of 1 mn or less), which are required in achievement of a high bite rate optical wavelength multiplexed transmission.
The abovementioned arrayed waveguide grating is an optical waveguide circuit that is formed so that an optical waveguide portion 10 having a lower cladding, a core and an upper cladding, which are formed of silica-based glass, is formed on a substrate 11 made of silicon, etc. The arrayed waveguide grating is such that the lower cladding is formed on the substrate 11 made of silicon, etc., the core which constitutes the abovementioned waveguide is formed thereon, and the upper cladding is further formed on the core so as to cover the core. In addition, the upper cladding was made of silica-based glass to which, B2O3 and P2O5 are, respectively, doped on pure silica glass at a ratio of 5 mole %.
FIG. 7 shows a manufacturing process of the arrayed waveguide grating. Hereinafter, a description is given of the manufacturing method of an optical waveguide circuit with reference to the same drawing. First, as shown in the same drawing (a), a film of the lower cladding 1b and a film of the core 2 are formed on the silicon substrate 11 in order. Next, as shown in the same drawing (b) , photolithography and reactive ion etching method are applied thereto, using a mask 8. By the application, as shown in the same drawing (c), an optical waveguide pattern of the arrayed waveguide grating is formed by processing the film of the core 2, whereby the core 2 of the optical waveguide construction is formed.
Next, as shown in the same drawing (d) , a film of the upper cladding 1a is formed on the upper side of the core 2 in a form embedding the core 2. In addition, the film of the upper cladding 1a is formed by depositing the upper cladding glass particles 5 by the flame hydrolysis deposition method and consolidating the upper cladding glass particles 5 at, for example, 1200xc2x0 C. through 1250xc2x0 C.
However, originally, in an arrayed waveguide grating that is applied as optical transmission elements for optical wavelength multiplexed transmissions as described above, it is preferable that the polarization dependency loss (PDL) in the TE mode and TM mode is as close to zero as possible. However, in the prior art arrayed wavelength grating, the abovementioned polarization dependency loss (PDL) was 3 dB where the center wavelength xcexc is in a range of xc2x10.1 nm.
Therefore, in order to compensate the polarization dependency loss, the prior art arrayed waveguide grating was constructed as shown in FIG. 8. That is, a half-wave plate 3 formed of polyimide, etc., is inserted into the middle way of the arrayed waveguide 14 in the form of crossing all the arrayed waveguides 14. If so, a polarized wave propagating through the arrayed waveguide grating is turned by 90 degrees at the incidence side and the emission side of the half-wave plate 3, and influences resulting from the abovementioned polarization dependency loss can be evaded. Also, the material of the half-wave plate 3 is not limited to polyimide, but it may be made of a silica-based glass material. However, since, if it is made of polyimide, the thickness thereof can be made thinner, a polyimide-made half-wave plate was most excellent as the half-wave plate 3 in the prior art arrayed waveguide grating.
However, as described above, if the arrayed waveguide grating is formed by inserting a half-wave plate 3, a so-called return loss occurs, in which a part of the light incident into the half-wave plate 3 returns to the incidence side of the incidence waveguides 12. In a case where the half-wave plate 3 is inserted vertical to the arrayed waveguides 14, the value of the return loss becomes approx. xe2x88x9235 dB. Thus, if a return loss exceeding xe2x88x9240 dB is produced in elements used for an optical wavelength multiplexed transmission system, the arrayed waveguide grating will not be able to be used for optical waveguide multiplexed transmissions.
In addition, in a case where the half-wave plate 3 is inclined unvertical by 8 degrees with respect to an axis vertical to the arrayed waveguides 14, the return loss can be suppressed by around xe2x88x9240 dB. However, in this case, even though a thin half-wave plate 3 made of polyimide is applied, it will become difficult for a slit, through which the half-wave plate 3 is inserted, to be formed and for the half-wave plate 3 itself to be inserted in view of the technical aspect. Therefore, in this case, another problem occurs, that is, the yield of arrayed waveguide gratings is lowered due to insertion of the half-wave plate 3.
In addition, if an attempt is made to juxtapose arrayed waveguides 14 to each other since the length of a polyimide-made half-wave plate 3 utilized at present is approx. 8 mm, only 320 arrayed waveguides 14 can be disposed at most. Therefore, even though an attempt is made to increase the number of arrayed waveguides 14 in order to achieve an arrayed waveguide grating having a narrow interval in future, a limitation is produced in the number of arrayed waveguides 14, and it becomes difficult to meet the requirement. Also, if an attempt is made to lengthen the polyimide-made half-wave plate 3, the production yield of the half-wave plates 3 themselves is lowered, and in line therewith, production costs of the arrayed waveguide gratings will be increased.
Still further, since a half-wave plate 3 is inserted into and fixed in an insertion slit worked at an arrayed waveguide grating by a dicer, etc., it becomes necessary to work the insertion slit in order to provide the half-wave plate 3. And, the half-wave plate 3 is inserted into the slit and is fixed with an adhesive agent, etc. For this reason, the number of processes of manufacturing arrayed waveguide gratings is increased, wherein still another problem occurs, that is, production costs of the arrayed waveguide grating are accordingly increased.
The present invention was developed to solve the abovementioned problems, and it is therefore an object of the invention to provide an optical waveguide circuit such as an arrayed waveguide grating having characteristics shown in (1) through (3) below: (1) an optical waveguide circuit provided by the invention can be manufactured by a fewer number of processes, and production costs thereof are inexpensive, (2) an optical waveguide circuit provided by the invention is excellent in return loss characteristics, and can suppress influences of polarization dependency loss, and (3) an optical waveguide circuit provided by the invention can construct an arrayed waveguide grating in which 320 or more arrayed waveguides can be juxtaposed to each other at intervals of 25 xcexcm.
In order to achieve the objects, the invention employs the following construction and means to solve the problems. According to the first aspect of the invention, an optical waveguide is an optical waveguide portion made of a lower cladding, a core and an upper cladding formed of silica-based glass is formed on a silicon substrate, wherein a value B of birefringence occurring at said optical waveguide portion is |B|xe2x89xa65.34xc3x9710xe2x88x925.
According to the second aspect of the invention, an optical waveguide circuit is, in addition to the first aspect, where it is assumed that the thermal expansion coefficient of the upper cladding is xcex1g, and the thermal expansion coefficient of a silicon substrate is xcex1s, xcex1s xe2x88x922.0xc3x9710xe2x88x927xe2x89xa6xcex1gxe2x89xa6xcex1s+2.0xc3x9710xe2x88x927 is established.
According to the third aspect of the invention, an optical waveguide circuit is that said arrayed waveguide grating is constructed by an optical waveguide circuit according to said first or second aspect, wherein a first slab waveguide is connected to the emission side of one or more juxtaposed optical input waveguides; a plurality of juxtaposed arrayed waveguides having different lengths from each other, which propagate light introduced from said first slab waveguide, are connected to the emission side of said first slab waveguide; a second slab waveguide is connected to the emission side of said plurality of arrayed waveguides; a waveguide construction, in which a plurality of juxtaposed optical output waveguides are connected to the emission side of said second slab waveguide, is formed of a core; and a plurality of optical signals having different wavelength from each other, which are inputted from said optical input waveguides, are caused to propagate with a difference in phase secured per wavelength by said arrayed waveguides, and are inputted into optical output waveguides differing per wavelength; whereby an arrayed waveguide grating is constructed, which outputs light having wavelengths different from each other through different optical output waveguides.
For example, in a case where the arrayed waveguide grating is applied to an optical wavelength multiplexed transmission system, it is preferable that the polarization dependency loss (PDL) in the TE mode and TM mode, being a polarization mode, is as close to zero as possible. Therefore, the inventor examined to which value the PDL of the arrayed waveguide grating is set for adequate optical wavelength multiplexed transmissions. As a result, it was found it was preferable that the PDL was set to 0.5 dB or less. And, in order to make the PDL smaller than 0.5 dB, the inventor also examined through simulation experiments to which value the quantity of center wavelength shift in the TE mode and TM mode in the arrayed waveguide grating is adequately set.
In the examination, first, optical transmission spectra in the respective polarization modes (TE mode and TM mode) of the arrayed waveguide grating were, respectively, measured. Also, it was assumed that the maximum value in difference between the transmission loss in the TM mode at the center wavelength xc2x1xcex2 (xcex2 is a predetermined value, for example, 0.8 nm) in the TE mode and the transmission loss in the TM mode at the center wavelength xc2x1xcex2 in the TM mode is a polarization dependency loss. And, for example, the transmission spectrum in the TM mode is caused to gradually shift in a suspected state with the transmission spectrum in the TE mode fixed, and the quantity of the shift was determined to be the center wavelength shift in the polarization mode. The polarization dependency loss is plotted with respect to the quantity of center wavelength shift to draw a graph. And, on the basis of the graph, the relationship between the quantity of center wavelength shift by the polarization mode and the polarization dependency loss was obtained.
The result is shown in FIG. 2. As shown in the same drawing, by setting the quantity of center wavelength shift in the TE mode and TM mode, which is the polarization mode in the arrayed waveguide grating, to 0.05 nm or less, it could be confirmed that the abovementioned PDL was made smaller than 0.5 dB. Therefore, the inventor determined that, in the case where an arrayed waveguide grating is used as an optical wavelength multiplexing transmission, an adequate shift quantity AXB of the center wavelength in the TE mode and TM mode is 0.05 nm or less.
Also, in an optical waveguide circuit such as an arrayed waveguide grating, etc., it is considered that the shift in the optical transmission center wavelength by the abovementioned polarization mode is influenced by a value of birefringence occurring in the optical waveguide portion. The quantity xcex94xcexB of center wavelength shift by the polarization mode can be expressed by the following equation, using a value B of birefringence occurring in the optical waveguide portion, a effective refractive index nc of the core, and center wavelength xcexO of light propagating through the core:
xcex94xcexB=|Bxc2x7xcexO/nc|xe2x80x83xe2x80x83(1)
Herein, in order to find out the optimal birefringence value in the case where an arrayed waveguide grating is applied to an optical wavelength multiplexed transmission system which is now under consideration, if xcexO and nc are determined as follows, the range of value B of birefringence occurring in the optical waveguide portion was obtained. And, the xcexO was determined to be 1550 nm which is the center wavelength in the optical wavelength multiplexed transmission system under consideration. Also, the refractive index of the core is determined by a composition of the core, and is determined to be approximately 1.45.
Where the value B of the birefringence is set in a range of |B|xe2x89xa65.34xc3x9710xe2x88x925, it was found that the value of xcex94xcexB of the quantity of center wavelength shift by the polarization mode could be set in the range. That is, it was found that, by setting the value B of the birefringence, the polarization dependency loss could be determined in the loss range required for an arrayed wavelength grating.
On the basis of the results of the abovementioned examinations, in the optical wavelength circuit according to the invention, the value B of the birefringence occurring in the optical waveguide portion is determined to be an adequate value of |B|xe2x89xa65.34xc3x9710xe2x88x925. In the other words, the stress provided onto the core by the stress applied from the cladding onto the core is made smaller. Accordingly, in the invention, the quantity of center wavelength shift in the abovementioned TE mode and TM mode in an optical waveguide circuit is established to be a value of 0.05nm or less, and the value of the polarization dependency loss (PDL) can be set to an adequate value of 0.5 dB or less.
Therefore, if an optical waveguide circuit according to the invention is applied to an optical wavelength multiplexed transmission system having, for example, a wavelength band of 1.55 xcexcm, it is possible to provide an optical waveguide circuit that can suppress influences of the polarization dependency loss without providing any half-wave plate. In further detail, as in the third construction of the invention, where an optical waveguide circuit is applied to an optical wavelength multiplexed transmissions system having a wavelength band of 1.55 xcexcm as an arrayed waveguide grating, it is possible to construct an arrayed waveguide grating that is capable of suppressing influences of the polarization dependency loss without providing any half-wave plate.
And, in the arrayed waveguide grating, the number of manufacturing processes can be accordingly reduced since no half-wave plate is required, whereby yield thereof can be increased to accordingly reduce the production cost thereof. In addition, the arrayed waveguide grating can be made so that it can accommodate, for example, 320 or more arrayed waveguides at internals of 25 xcexcm, wherein it is possible to increase the number of channels.
Also, in the invention, by establishing xcex1s, xcex1sxe2x88x922.0xc3x9710xe2x88x927xe2x89xa6xcex1gxe2x89xa6xcex1s+2.0xc3x9710xe2x88x927 where it is assumed that the thermal expansion coefficient of the upper cladding is xcex1g, the thermal expansion coefficient of the upper cladding can be optimized.
Therefore, if an optical waveguide circuit according to the invention is thus constructed, it is possible to obtain an optical waveguide circuit that can bring about the abovementioned excellent effects.